Measurement of shear strength and texture evolution in BCC materials subjected to high pressures

Deformation modeling of metals subjected to extreme pressures and strain rates requires an understanding of the pressure-dependent dislocation core structure and its effect upon dislocation mobility. The core structure and dislocation mobilities can be predicted as a function of applied pressure from sophisticated interatomic potentials calculations and first-principles based atomistic simulations.;The goal of the thesis is to develop and implement a testing procedure that experimentally determines pressure-dependent dislocation mobilities in oriented single crystals of the BCC transition metals. These experiments provide calibration data for models of materials subjected to extreme pressures and assist in model validation such as the Steinberg-Guinan hardening model or discrete dislocation dynamics simulations. An experimental procedure is reported to perform shear tests on specimens held under moderately high hydrostatic pressures (on the order of 10 GPa). A thin foil of polycrystalline Ta was used to perform experiments under hydrostatic pressures ranging from 2.1 to 4.2 GPa. A change in texture due to accumulation of slip was observed. Close to a strain of 1, the texture is predicted to change from {111} + {100} to {101}+{121}+{123}, the primary and secondary slip planes in BCC. These {101}+{121}+{123} textures were present in all the samples subjected to pressures greater than the threshold pressure to have internal shearing. The experimental (TEM) evidence shows different microstructures with the pressure being the only variable. At low pressures (2 GPa), an expected microstructure containing only dislocations was found to be responsible for the plastic deformation. At higher pressures (4 GPa) the dislocations appear to arrange themselves into elongated cell walls, with widths of 50-100 nm and lengths close to a micron.;Testing on Mo single crystals were carried out. Two different orientations {110}<111> and {121}<111> were tested such that simple shear deformation was achieved by single slip on a single slip system with no additional slip activity. The experiments provided data on the shear-stress---shear strain behavior of the single crystals as a function of pressure, giving an indication of dislocation mobility on the given slip system.;In all the cases, yielding and hardening behavior were observed to be sensitive to the imposed pressure. The values obtained experimentally are considerably higher than those predicted by the models based on a linear pressure-dependent shear modulus G(P).